1. Field of the Invention
The present invention relates to a gas flow sensor which includes resistors on a substrate member, which develop voltages thereacross in correspondence to the mass of gas to be measured on the basis of the differences in electric resistance resulting from the differences in temperature produced by the transfer of heat by said gas to be measured passing through a passage of said gas to be measured as a heat transfer medium to which heat energy is given, and a method of manufacturing same.
2. Description of the Prior Art
The above described gas flow sensor aims at the measurement of a mass flow rate (hereinafter "flow rate") of a gas to be measured passing through a passage and is used for example in an automatic air-to-fuel ratio control system installed in a passage of a combustion air for an engine for measuring the quantity of combustion air passing through said passage and simultaneously feeding the desired quantity of fuel in a cylinder of an engine on the basis of the measured quantity of a combustion air.
Although there are two types of the above described gas flow sensor: a self-heating type one, in which two heating resistors are arranged with an interval therebetween in the direction of gas flow, and an auxiliary heating type one, in which one heating resistor and two heat sensitive resistors respectively upstream and downstream of said heating resistor in the direction of gas flow are arranged with intervals therebetween. In every case, the resistors are connected with a bridge circuit.
In the former self-heating type sensor, heat energy is given to a gas to be measured from a heating resistor at an upstream side of the sensor in the passage and said gas to be measured flowing in the passage is used as a heat transfer medium for transporting the heat energy to a heating resistor at the downstream side of the sensor. A bridge circuit is electrified to heat said heating resistors, and said gas to be measured is passed through said passage. As a result, the electric resistance of the upstream heating resistor is reduced while the electric resistance of the downstream heating resistor is increased to break the equilibrium in said bridge circuit and the voltage is developed in correspondence to the flow rate of said gas on the basis of a difference in electric resistances. On the other hand, in the latter auxiliary heating type sensor, both heat sensitive resistors are only slightly electrified and a heating resistor is heated by electrifying it, whereby transferring heat energy of the downstream heating resistor to said heat-sensitive resistor. The voltage is developed in correspondence to the flow rate of a gas to be measured on the basis of the difference in electric resistances of between the heat sensitive resistors owing to the above described transfer of heat.
Such a flow sensor has been widely used in the semi-conductor industry, the automotive industry, the analytical instrument industry and industry in general as a sensor for use in a mass flow meter for accurately measuring the mass flow rate of fluid and a sensor for use in a mass flow controller for accurately controlling the mass flow rate in addition to a sensor for use in an automatic control of an air-to-fuel ratio. Since the accurate measurement and control of the flow rate of gases used as the raw materials in the manufacture of semi-conductors have a serious influence upon the quality and yield of the resulting semi-conductors, such a flow sensor has been used in the semi-conductor manufacturing apparatus in great quantities. Furthermore, also in the fields where such a sensor has not been used, there has been an increase in the use of mass flow controllers.
Although the provision of an inexpensive and uniform in characteristics gas flow sensor is desired, the above described desire has not been satisfied for the present.
That is to say, although a hot wire type gas flow sensor has been frequently used for a gas flow sensor for use in controlling air-to-fuel ratio and it has been manufactured by winding a resistor made of platinum wire around a sensor substrate, there are limits to the automatization of the operation of winding platinum wires around each sensor substrate and the manufacture of a gas flow sensor cannot but be dependent upon manual operations. As a result, the product cost is increased since the productivity is low and it is difficult to mass-produce uniform in characteristics products.
In addition, the stream of gas is apt to be disordered causing a reduction in the accuracy of measurement since the column-like structure of the sensor is inserted into the stream of gas.